The present invention relates to a bit holding system for an electric hand tool for example an electric hand drill, and more particularly to an auto-locking bit holding system which automatically locks the bit when the electric hand tool is operated.
FIG. 1 shows a bit holding system for an electric hand tool according to the prior art. This structure of bit holding system comprises a hollow cylindrical transmission block having a rear end thereof fixedly connected to one end of the transmission shaft of the electric hand tool and a plurality of through holes spaced around the periphery, a plurality of clamping rods respectively obliquely inserted through the through holes on the hollow cylindrical transmission block and forced to hold down the bit being inserted into the hollow transmission block, the clamping rods each having a threaded portion at an outer side outside the hollow cylindrical transmission block, a nut having a tapered screw hole threaded onto the threaded portions of the clamping rods, a shell covered around the hollow cylindrical transmission block and the nut, and a locating barrel fastened to the shell to stop the nut inside the shell. When the shell is compressed with the hand and then turned, the nut is turned with the shell relative to the clamping rods to hold down or release the clamping rods. This structure of bit holding system is still not satisfactory in function. This structure of bit holding system is that the shell is not convenient in use because the shell must be turned by hand when holding down or releasing the clamping rods. Another drawback of this structure of bit holding system is that iron chips from the workpiece tends to be gathered at the shell because the shell is rotated with the transmission block during the operation of the electric hand tool, therefore the user's hand tends to be injured by iron chips gathered at the shell. Furthermore, because vibration waves are transmitted from the bit to the clamping rods, the transmission block and the nut during the operation of the electric hand tool, the tapered screw hole of the nut and the threaded portions of the clamping rods tend to be damaged.